1. Field of the Invention
The present invention relates to a method of compensating for the effects of motion of an image subject during magnetic resonance imaging (MRI) using phase encoding in a magnetic resonance imaging system.
2. Description of the Prior Art
Current MRI systems (also known as MRI scanners) employ arrays of local radio frequency (RF) receiver coils mounted in close proximity to the scanned patient to receive the RF with maximum possible signal to noise ratio (SNR). The coils that receive signals from the lower side of the patient are mounted in a patient table, upon which the patient lies. RF receiver coils that receive signals from the upper side of the patient are typically arranged into ‘blankets’ that are placed over the patient during imaging. The blanket is typically connected to a flexible cable containing one co-axial line for each RF receiver coil. The cables may interact with the rotating RF magnetic field (B1) and with the RF signals emitted due to magnetic resonance within the patient. In an attempt to mitigate these interfering effects, it is known to provide high impedance sections, known as ‘traps’ at regular intervals, typically λ/8, where λ is the wavelength of the RF signals of interest. The traps add cost and inconvenience to the structure.
In use, the requirement to connect the cables and sterilize them between scanning one patient and the next leads to increased down-time between scans. It is therefore very attractive to develop a concept that permits the cables to be eliminated by a wireless solution. Ideally, the wireless solution substantially satisfies all the requirements of the existing system, particularly with regard to noise.
The wireless coils concept involves upconverting the Larmor frequency MR signal received by the local coils to a much higher frequency in the 2.4 GHz band. The upconverted signal is transmitted across a short radio path to an array of receive antennas and receivers that line the bore. Movement of the patient during a scan will vary the length of the radio paths, resulting in changes in amplitude and phase. The local oscillator frequency is much higher than the Larmor frequency, (typically about 40 times for a 1.5 T scanner), so the effect of the movement on phase is greatly magnified. For example, at 2.45 GHz, a 5 mm change in path length corresponds to a phase shift of 15°. However, the effect is greater than this, because the upconversion is performed using a local oscillator signal that has been transmitted from the bore antennas to the patient. Thus, the change in path length will also alter the local oscillator signal path. The combined effect is therefore to double the impact of patient movement—thus a 5 mm change in path length will correspond to a phase shift of 30°.